Since I've gone from b2 to b3 I've had strange slowdowns on Vista 64, for instance when switching from a drive to another.

More over, today I opened the program and it closed after a few seconds, saying the executable was corrupted and to check if I had a virus. I scanned the pc with both online and offline antiviruses and found nothing.

I think Avira Antivir has some problem with the b3 executable though, and that may be the cause of the slowdown. If I scan the TC folder with old versions up to b2, it takes about half a second to scan. If I scan with b3, it takes about 4 minutes, getting stuck for two minutes on history.txt and totalcmd.exe.

Quite strange, I know... I was almost positive it was a virus, but it doesn't get detected in any way.

I cannot confirm either of the two things which you complain about:
+ Total Commander 7.50 PB3 does not run any slower than the previous versions
+ Avira Antivir has not got any problem with Total Commander 7.50 PB3.

As you seem to experience quite a different behaviour of Avira, I assume that either of the following factors may make the difference:
+ You use a different Avira version
+ Your Avira configuration is pretty different from mine
+ Your Avira malware definition files need an update urgently

In any case, if you really think that Avira is to be blamed you should consider reporting so to Avira in the first place.

I really fail to see how a suspected slowdown caused by Avira might constitute a T.C. 7.50 PB3 bug?

My Avira is the Premium version. I'll try to report the problem to them. But I noticed that after reinstalling b3 the problem is gone... I really don't know how the TC executable became corrupted, since everything else keeps on working just fine.